Semiconductor device with a high thermal dissipation efficiency

ABSTRACT

A semiconductor device having a higher thermal dissipation efficiency includes a thermally conducting structure attached to a surface of the semiconductor device via soldering. The thermally conducting structure is essentially formed of a thermally conducting material and comprises an array of freestanding fins, studs or frames, or a grid of connected fins. A process for fabricating such a semiconductor device includes forming a thermally conducting structure on a carrier and attaching the thermally conducting structure formed on the carrier to a surface of the semiconductor device via soldering.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of, and claims priority from,commonly-owned and co-pending U.S. patent application Ser. No.11/852,317, filed on Sep. 9, 2007, which is incorporated by reference asif fully set forth herein.

STATEMENT REGARDING FEDERALLY SPONSORED-RESEARCH OR DEVELOPMENT

None.

INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

None.

FIELD OF THE INVENTION

The present invention relates to a semiconductor device with higherthermal dissipation efficiency and to a method for fabricating the same.

BACKGROUND OF THE INVENTION

In order to meet the demand for higher clock frequencies of futurecomputers, the integration density of current semiconductor devices as,for example, chips and multi-chip modules (MCM), in which a number ofsemiconductor chips are mounted on a common substrate generally in theface-down state, is continuously being increased. As a consequence,there arises the problem of increased heat dissipation due to theincrease in power density.

Common cooling approaches include air cooling, as for example disclosedin the U.S. Pat. No. 5,471,366, which describes a multi-chip module anda fabrication process thereof. The multi-chip module comprises aplurality of semiconductor chips mounted on a common substrate and aplurality of thermally conductive blocks attached to the semiconductorchips. A resin package body encapsulates the semiconductor chips and thethermally conductive blocks together with the substrate. The resinpackage body furthermore has an upper surface flushing with the uppersurfaces of the thermally conductive blocks. A heat sink carrying heatradiation fins is mounted onto the upper surface of the resin packagebody in such a way that a thermally contact is established between theheat sink and the upper surfaces of the thermally conductive blocks.Thus, heat dissipated from the semiconductor chips is transferred to theheat sink via the thermally conductive blocks and is radiated to thesurrounding air from the heat radiation fins.

One disadvantage of this multi-chip module is that the heat transferredfrom the semiconductor chips to the air has to pass three boundarylayers, so that the heat dissipation efficiency is reduced. Besides, there-workability in case of defects or insufficient thermal connections isrestricted due to the encapsulation of the thermally conductive blocks,the semiconductor chips and the substrate with the resin package body.

Furthermore, in general air cooling approaches are quickly reachingtheir limits. As a consequence, alternative cooling concepts have to bepursued.

Other known cooling techniques include liquid coolants, which will becrucial for future midsize and portable computers in particular. Thedisadvantage of these cooling methods consists in the connection ofliquid coolers to the surfaces of the semiconductor chips. In order tolevel out roughness and warp, a thick thermally interface between thechips and the liquid coolers is required which causes a higher thermallyresistance that deteriorates the cooling performance.

One of the currently used cooling methods is the so-called jetimpingement cooling. The idea behind this cooling method is to spray acooling fluid through nozzles directly onto the backside surface of thesemiconductor chips in order to create a film of fluid thereon.Consequently, heat transfer to the cooling fluid is rendered easier.However, the cooling efficiency of jet impingement coolers is generallylow due to the unstructured only flat impingement surface of thesemiconductor chips.

Moreover, it is known to etch trenches into the backside of a siliconwafer which serve as microscopic fluid channels in the futuresemiconductor chips. This concept of micro-channel heat sinks is basedon very short thermally conduction paths and a high surface enlargementfactor and utilizes the good thermally conductivity of silicon. It hasbeen shown that the highest cooling efficiencies are possible withchannels having a width of 30 to 50 μm an and a depth of 200 to 300 μm.

A drawback of this method is that surface enlargement of processor chipsby deep trench etching of wafers causes yield reduction, either due tothe deep trench etch process stability or due to mechanical fracture ofthe patterned surface. Therefore, the method is cost ineffective.Moreover, deep trench backside etching of wafers can cause processcompatibility issue problems, does not allow a re-working of thisprocess in case of defects and makes the semiconductor chips morefragile to breaking. As a consequence, such etching processes are notaccepted in chip manufacturing.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novelsemiconductor device having a higher thermal dissipation efficiency anda good mechanical stability, which is compatible to the existingsemiconductor process technology and which provides the possibility ofre-working in case of defects.

Another object of the present invention is to provide a method forfabricating a semiconductor device having a higher thermal dissipationefficiency with minimal induction of stress to the semiconductor device(well below fracture strength of the device), even if differentmaterials are used.

According to one aspect of the present invention, a semiconductor devicecomprising a thermally conducting structure attached to a surface of thesemiconductor device via soldering is provided. The thermally conductingstructure is essentially formed of a thermally conducting material andcomprises an array of freestanding fins, studs or frames, or a grid ofconnected fins.

Due to the thermally conducting structure, the surface of thesemiconductor device is enlarged. Since the solder can be maderelatively thin, for example less than 1 μm, the solder does not limitthe thermally conduction from the semiconductor device to the thermallyconducting structure. Consequently, the semiconductor device provides ahigher thermal dissipation efficiency. Moreover, the semiconductordevice has a good mechanical stability due to the absence of etchedareas on the surface. In addition, the semiconductor device allows forre-working in case of defects by re-melting of the solder, removing thethermally conducting structure or a portion thereof and attaching a newthermally conducting structure or a portion thereof to the surface ofthe semiconductor device via soldering.

In a preferred embodiment of the present invention, the semiconductordevice further comprises a manifold layer attached to the thermallyconducting structure. By using a manifold layer, it is possible tosupply a cooling liquid or water to the structural elements of thethermally conducting structure. Thereby, a very higher coolingefficiency of the semiconductor device can be obtained.

In another preferred embodiment of the present invention, thesemiconductor device is integrated into a device for jet impingementcooling. Due to the fact that the surface of the semiconductor device isenlarged by the thermally conducting structure, a good coolingperformance of the semiconductor device can be achieved.

According to another aspect of the present invention, a method forfabricating a semiconductor device is provided. In a first step, acarrier having a seed layer is provided. A patterned mask layer isprovided on the seed layer of the carrier afterwards, wherein thepatterned mask layer has a recess structure. A thermally conductingmaterial is deposited on the patterned mask layer to fill up the recessstructure of the patterned mask layer, thus forming a thermallyconducting structure. Subsequently, a solder is deposited on thethermally conducting structure. After that, the patterned mask layer andthe seed layer between the structural elements of the thermallyconducting structure are removed and the thermally conducting structureformed on the carrier is attached to a surface of a semiconductor devicevia soldering. After joining the thermally conducting structure getsreleased.

As described above, the thermally conducting structure allows for anenlargement of the surface of the semiconductor device and the soldercan be rendered thinner that the thermally conduction from thesemiconductor device to the thermally conducting structure is improved.Consequently, the method makes it possible to produce a semiconductordevice with a higher thermal dissipation efficiency by forming,attaching and transferring a thermally conducting structure, e.g. anisland or a high-aspect ratio structure such as an array of freestandingstuds, to the semiconductor device. Due to the fact that the thermallyconducting structure is formed on the carrier and thus separately fromthe semiconductor device, the used process steps do not have to becompatible with the manufacturing of the semiconductor device. Moreover,the method provides the re-workability in case of defects merely byremoving the thermally conducting structure or a portion thereof fromthe surface of the semiconductor device via re-melting of the solder andattaching a thermally conducting structure or a portion thereof formedon another carrier to the surface of the semiconductor device.

In a preferred embodiment of the present invention, the further step ofplanarizing the patterned mask layer and the thermally conductingstructure after said step of depositing the thermally conductingmaterial is introduced in order to achieve a planar surface of thepatterned mask layer and the thermally conducting structure.

In another preferred embodiment of the present invention, a further stepof coating the surface of the semiconductor device with an adhesionlayer prior to said step of attaching the thermally conducting structureformed on the carrier to the surface of the semiconductor device isintroduced. As a result, a bigger variety of possible solder materialsis available in order to attach the thermally conducting structure tothe semiconductor device.

In another preferred embodiment of the present invention, the carrier isa manifold layer. Consequently, the semiconductor device can be providedwith a liquid coolant, thus achieving a higher cooling efficiency of thesemiconductor device.

In another preferred embodiment of the present invention, the methodincludes the step of providing the carrier as a transparent substratecoated with a polyimide layer, wherein the seed layer is being formed ontop of the polyimide layer.

In this connection, it is preferred to remove the carrier after saidstep of attaching the thermally conducting structure formed on thecarrier to the surface of the semiconductor device and to integrate thesemiconductor device with the thermally conducting structure attached tothe same into a device for jet impingement cooling. Since the surface ofthe semiconductor device is enlarged by the thermally conductingstructure, a good cooling performance of the semiconductor device can beachieved.

The present invention can in particular be used to fabricate amulti-chip module having a higher thermal dissipation efficiency.Accordingly, in yet another preferred embodiment of the presentinvention, the semiconductor device comprises at least two semiconductorchips attached to a common substrate. Thereby, the method includes thestep of attaching the thermally conducting structure to the surfaces ofthe semiconductor chips.

Concerning the term “surface” of the semiconductor device, the backsideof the semiconductor device, that is defined as the side that comprisesno device components, is preferred because the processing steps are lesslikely to harm the components and influence their functionality.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

These and other objects and features of the present invention willbecome clear from the following description taken in conjunction withthe accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a multi-chip module with twosemiconductor chips according to a first embodiment of the presentinvention, which is integrated into a device for jet impingementcooling;

FIG. 2 is a plan view of a semiconductor chip of the multi-chip moduleof FIG. 1;

FIGS. 3A to 3F illustrate a fabrication process of the semiconductorchip of FIG. 2 in a cross-sectional view;

FIG. 4 is a cross-sectional view of a multi-chip module with twosemiconductor chips according to a second embodiment of the presentinvention, which comprises a manifold layer;

FIG. 5 is a plan view of a semiconductor chip of the multi-chip moduleof FIG. 4;

FIG. 6 is a plan view of the manifold layer of the multi-chip module ofFIG. 4;

FIG. 7 is a plan view of a semiconductor chip of the multi-chip moduleof FIG. 4 according to an alternative embodiment of the presentinvention;

FIGS. 8A to 8H illustrate the fabrication process of a multi-chip moduleaccording to a third embodiment of the present invention, whichcomprises two semiconductor chips and two manifold layers attached tothe same;

FIG. 9 is a plan view of the manifold layers of the multi-chip module ofFIG. 8H;

FIG. 10 is a cross-sectional view of the multi-chip module of FIG. 8H,which comprises an additional connection layer mounted on top of themanifold layers;

FIG. 11 is a plan view of the connection layer of the multi-chip moduleof FIG. 10;

FIGS. 12A to 12G illustrate an alternative fabrication process of themulti-chip module of FIG. 8H;

FIG. 13 is a schematic cross-sectional view of a part of a manifoldlayer attached to a stud via a spring element;

FIG. 14 is a plan view of the part of the manifold layer attached to thestud via the spring element; and

FIG. 15 is a cross-sectional view of a semiconductor chip comprising anarray of T-shaped fins attached to the backside of the same.

While the invention as claimed can be modified into alternative forms,specific embodiments thereof are shown by way of example in the drawingsand will herein be described in detail. It should be understood,however, that the drawings and detailed description thereto are notintended to limit the invention to the particular form disclosed, but onthe contrary, the intention is to cover all modifications, equivalentsand alternatives falling within the scope of the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a multi-chip module 1, according to a first embodiment ofthe present invention in a cross-sectional view, which is integratedinto a device for jet impingement cooling 13. The multi-chip module 1comprises a common substrate 3 carrying a multilayer interconnectionstructure thereon and two semiconductor chips 2 attached to the surfaceof the substrate 3 in a face-down state via solder balls 4. Thesemiconductor chips 2 are mounted onto the surface of the substrate 3 bya flip chip process such as the common C4 technology process.

Each of the semiconductor chips 2 comprises an adhesion layer 5 coatingthe backsides of the semiconductor chips 2. The adhesion layers 5consist for example of nickel and gold. The multi-chip module 1 furthercomprises a thermally conducting structure comprising an array offree-standing studs 7 attached to the adhesion layers 5 of thesemiconductor chips 2 via solder bonds 6. The studs 7 are essentiallyformed of a thermally conducting material such as copper and preferablyhave a height of about or more than 100 μm. By means of the studs 7, thesurface of the backsides of the semiconductor chips 2 is enlarged.

The solder bonds 6 have a thickness of about or less than 1 μm. Theadhesion layer 5 has a thickness of about 1 μm. As a consequence, athermally conduction from the backsides of the chips 2 to the studs 7 isimproved. The device for jet impingement cooling 13 comprises nozzles 14in order to spray a cooling fluid 16 onto the backsides of thesemiconductor chips 2. Due to the fact that the backside surface of eachchip 2 is enlarged by the array of studs 7, a higher cooling performanceis obtained. The fraction of cooling fluid 16 that evaporates or getsheated up and stays liquid during the cooling process exhausts throughan outlet 15.

FIG. 2 depicts a plan view of a semiconductor chip 2 of the multi-chipmodule 1 of FIG. 1. From this view, the array of free-standing studs 7joined with the backside of the chip 2 can be seen. In an alternativeembodiment, the semiconductor chip 2 may also comprise a thermallyconducting structure with a different shape which consists for exampleof an array of free-standing fins attached to the backside thereon (notshown). Next, the fabrication process of the semiconductor chip 2 ofFIG. 2 will be described with references to FIGS. 3A to 3F. In a firststep, a carrier 8 is provided having a seed layer 11, as indicated inFIG. 3A. The carrier 8 comprises a transparent substrate 9 of e.g. glasswhich is coated with a polyimide layer 10. The seed layer 11 thatcomprises an electro-conductive material is being formed on top of thepolyimide layer 10.

Thereafter, a mask layer 12, e.g. photoresist, is deposited on the seedlayer 11 and patterned, thus forming a recess structure in the masklayer 12. Further, a thermally conducting material such as copper isdeposited on the patterned mask layer 12 in order to fill up the recessstructure of the patterned mask layer 12. The depositing of thethermally conducting material is e.g. done by electroplating. As aresult, a thermally conducting structure 7 comprising an array of studs7 is formed by the thermally conducting material in the recess structureof the patterned mask layer 12, as shown in FIG. 3B.

Afterwards, the patterned mask layer 12 and the thermally conductingstructure 7 are planarized to achieve a planar surface of the patternedmask layer 12 and the thermally conducting structure 7, as indicated inFIG. 3C. This step is e.g. carried out by chemical and mechanicalpolishing (CMP). Thereafter, a solder 6 is deposited on the studs of thethermally conducting structure 7, as shown in FIG. 3D. This can beachieved by electroplating, sputtering or evaporation. In the case ofsputtering and evaporation, an additional patterned mask layer (notshown) is provided on the patterned mask layer 12 and removed after thesolder depositing.

After that, the patterned mask layer 12 and the seed layer 11 areremoved between the studs 7 of the thermally conducting structure, asshown in FIG. 3E. This is e.g. performed by an etching process.

Afterwards, the studs 7 formed on the carrier 8 are solder-bonded andtransferred to the backside of the semiconductor chip 2 which has beencoated with an adhesion layer 5. At the end, the carrier 8 is removed bye.g. laser ablating of the polyimide layer 10, so that the semiconductorchip 2 with an array of free-standing studs 7 attached to the backsideof the chip 2 is obtained, as indicated in FIG. 3F. Thereby each stud 7is covered by the seed layer 11. For reasons of simplicity, the seedlayer 11 covering the studs 7 has been left out in FIG. 1.

As the studs 7 are formed on the carrier 8 separately from thesemiconductor chip 2 and the thermally conducting structure area issmall, the described fabrication process is characterized by no orlittle mechanical stress induction to the semiconductor chip 2, thusallowing an enlargement of the backside surface of the chip 2 withoutgreat changes in chip backend processing, even by using differentmaterials with even better properties than silicon. Furthermore, thesemiconductor chip 2 with the studs 7 can be re-worked in case ofdefects by simply re-melting of the solder bonds 6, removing the studs 7or a portion thereof from the backside of the semiconductor chip 2 andattaching newly formed studs 7 with a carrier 8 to the same viasoldering.

In order to obtain the multi-chip module 1 as depicted in FIG. 1, twosemiconductor chips 2 are processed according to the fabrication processshown in FIGS. 3A to 3F and are subsequently mounted onto the surface ofa common substrate 3 by, for example, a standard C4 technology process.Afterwards, the multi-chip module 1 is integrated into a device for jetimpingement cooling 13.

Alternatively, the semiconductor chips 2 can be attached to thesubstrate 3 at first, and afterwards studs 7 formed on separate carriers8 or one common carrier are transferred and bonded to the backsides ofthe semiconductor chips 2.

In contrast to the multi-chip module 1 depicted in FIG. 1 and thesemiconductor chip 2 shown in FIG. 3F, respectively, the studs 7 canalso be attached to the backside of the semiconductor chip 2 which isnot coated with an adhesion layer 5. In such embodiments a reactivesolder material is used as the solder 6 which directly joins with thebackside of the semiconductor chip 2. However, the application of anadhesion layer 5 allows a bigger variety of solder materials which canbe used.

Instead of integrating a multi-chip module 1 into a device for jetimpingement cooling 13, the multi-chip module 1 and the semiconductorchips, respectively, can also be provided with micro channel or ductcooling. In this regard, FIG. 4 illustrates, in a cross-sectional view,a multi-chip module 20 according to a second embodiment of the presentinvention. This multi-chip module 20 also comprises two semiconductorchips 2 attached to the upper surface of a common substrate 3 in theface-down state via solder balls 4. The backsides of the semiconductorchips 2 are again each covered with an adhesion layer 5.

Instead of free-standing studs, the multi-chip module 20 furthercomprises two grids 21 of connected fins attached to each of thebacksides of the semiconductor chips 2 for surface enlargement. Thesegrids 21, which comprise a thermally conducting material such as copper,are bonded to the adhesion layers 5 coating the backsides of the chips 2via solder bonds 6.

For this, FIG. 5 depicts a plan view of a semiconductor chip 2 of themulti-chip module 20 of FIG. 4 with a grid 21 of connected fins. Thegrid 21 comprises a plurality of recesses 23 exposing the backsidesurface of the semiconductor chip 2.

As can be seen from FIG. 4, the multi-chip module 20 further comprises amanifold layer 30 attached to the grids 21 of the semiconductor chips 2via a sealing 36. The manifold layer 30 comprises an inlet 31 which canbe connected to a liquid or water source (not shown) and an outlet 33.As can be deduced from the plan view of the manifold layer 30 shown inFIG. 6, the manifold layer 30 further comprises an inlet channel 32, anoutlet channel 34 and a number of channels 35 extending from the inletchannel 32 and the outlet channel 34 towards the recesses 23 of thegrids 21. The inlet 31 is connected thereby with the inlet channel 32and the outlet 33 with the outlet channel 34.

In order to cool the backsides of the semiconductor chips 2, the inlet31 of the manifold layer 30 is connected to a water or liquid source(not shown). Thus, a cooling liquid 24 flows towards the recesses 23 ofthe grids 21 via the inlet 31, the inlet channel 32 and respective onesof the channels 35 of the manifold layer 30, which can be seen fromFIGS. 4, 5 and 6. The cooling liquid 24 then flows down into therecesses 23 of the grids 21, is warmed up and flows up as warmed coolingliquid 24 a, as can be seen from FIG. 4. Subsequently, the warmedcooling liquid 24 a flows through respective ones of the channels 35,the outlet channel 34 and the outlet 33 of the manifold layer 30, whichcan be seen from FIGS. 4 and 6.

The recesses 23 of the grids 21 preferably have a width of about 50 μmand a depth of about between 100 to 500 μm. Thus, the backside surfaceof the semiconductor chips is enlarged. In addition, the solder bonds 6and the adhesion layers 5 of the multi-chip module 20 have thicknessesof less or about 1 μm, so that the thermally conduction from thebacksides of the semiconductor chips 2 to the grids 21 of connected finsis improved. As a consequence, a higher cooling performance of theliquid or water-cooled multi-chip module 20 is realized.

The manifold layer 30 can comprise a rigid material like e.g. glassceramics. In order to reduce stress induction to the multi-chip module20 due to different thermal expansions of the manifold layer 30 and thesemiconductor chips 2, it is preferred that the manifold layer 30comprises the same material as the semiconductor chips 2, i.e. silicon.

In order to accommodate geometrical imperfections between thesemiconductor chips 2 and to enhance the structural flexibility of themulti-chip module 20, the manifold layer 30 comprises a thinning 37between the semiconductor chips 2 and between the inlet and outletchannels 32, 34, as indicated in FIGS. 4 and 6. These imperfections areadditionally compensated by the flexible sealing 36, which consists e.g.of polydimethylsiloxane (PDMS). The sealing 36 may also comprise anadhesive.

Alternatively, the manifold layer 30 can also comprise a flexiblematerial having a low Young's modulus like PDMS. Thus, geometricalimperfections between the chips 2 and different thermal expansions ofthe manifold layer 30 and the chips 2 can also be compensated.

Since each of the grids 21 is attached to the whole backside surface ofa respective one of the semiconductor chips 2, mechanical stress due todifferent thermal expansions of the grids 21 and the semiconductor chips2 can occur. In order to reduce this problem, the backsides of thesemiconductor chips 2 of the multi-chip module 20 can alternatively beprovided with an array of free-standing frames 22, as indicated in theplan view of a semiconductor chip 2 in FIG. 7. The frames 22, which alsocomprise recesses 23 for receiving the cooling liquid 24, form astructure similar to the grid 21. Thereby, the manifold layer 30 canalso be attached to the frames 22 via the sealing 36. Between the frames22, stress release gaps 25 are provided which allow a reduction ofmechanical stress induction caused by different thermal expansions of asemiconductor chip 2 and the frames 22.

Alternatively, the backsides of the semiconductor chips 2 of themulti-chip module 20 can also be provided with an array of free-standingstuds or fins (not shown), whereas the manifold layer 30 can be attachedto the studs or fins via a sealing. In such an embodiment, the coolingperformance achieved by liquid or water cooling is possibly better dueto a spreading and mixing of cooling liquid between the studs or fins aswell. The delivery rate of the liquid or water source connected to theinlet 32 of the manifold layer 30 is possibly higher, however, due to anincreased pressure drop between the studs or fins.

In order to produce a multi-chip module with a manifold layer, such asthe multi-chip module 20 depicted in FIG. 4, the fabrication processdescribed above with reference to FIGS. 3A to 3F can be utilized. Afterthe steps of removing the carrier and mounting the semiconductor chips 2onto a common substrate 3, a manifold layer is attached to the thermallyconducting structure, i.e. a grid of connected fins or an array offrames, studs or fins, which is attached to the backsides of thesemiconductor chips 2.

Instead of fabricating a multi-chip module by using a carrier comprisinga transparent substrate and a polyimide layer, as illustrated in FIGS.3A to 3F, a manifold layer can be used as carrier, as well. For this,FIGS. 8A to 8H illustrate the fabrication process of a multi-chip module40 according to a third embodiment of the present invention.

In a first step, a manifold structure or manifold layer 41 having anelectro-conductive seed layer 11 is provided, as shown in FIG. 8A.Afterwards, the manifold layer 41 is covered with a top sealing 42comprising holes 43 which serve as future inlet and outlet.

After that, a patterned mask layer 12, e.g. photoresist having a recessstructure is provided on the seed layer 11. Subsequently, a thermallyconducting material such as copper is deposited on the patterned masklayer 12 to fill up the recess structure of the patterned mask layer 12.This step is e.g. performed by electroplating. Thus, a thermallyconducting structure 21 is formed by the thermally conducting materialin the recess structure of the patterned mask layer 12 as can be seen inFIG. 8C. The thermally conducting structure 21 exhibits the geometricalshape of the grid 21 of connected fins, for instance, which is depictedin the plan view of FIG. 5. Alternatively, the thermally conductingstructure 21 could also comprise an array of frames, studs or fins.

Referring to FIG. 8D, the patterned mask layer 12 and the thermallyconducting structure 21 are planarized by e.g. chemical and mechanicalpolishing to achieve a planar surface of the patterned mask layer 12 andthe grid 21. Afterwards, a solder 6 is deposited on the grid 21 ofconnected fins, as illustrated in FIG. 8E. This step is for examplecarried out by electroplating.

In a next step, depicted in FIG. 8F, the patterned mask layer 12 and theseed layer 11 between the fins of the grid 21 are removed by, forexample, etching and the grid 21 of connected fins is transferred andsolder-bonded to the backside of a semiconductor chip 2. As shown inFIG. 8F, the backside of the semiconductor chip 2 is again coated withan adhesion layer 5. As discussed before, the adhesion layer 5 can alsobe omitted.

Referring to FIG. 8G, the front side of the semiconductor chip 2 isprovided with solder balls 4, which can be performed by a standard C4technology process. Afterwards, the semiconductor chip 2 depicted inFIG. 8G and a second chip 2 also fabricated in accordance with theprocess steps illustrated in FIGS. 8A to 8G are mounted onto the uppersurface of a common substrate 3 via soldering in order to obtain themulti-chip module 40, which is depicted in FIG. 8H.

Similar to the fabrication process described above with reference toFIGS. 3A to 3F, the fabrication process illustrated in FIGS. 8A to 8Hallows an enlargement of the backside surfaces of the semiconductorchips 2 with no or only little stress induction and without many changesin chip backend processing. The multi-chip module 40 or thesemiconductor chips 2 can also be re-worked if required by re-melting ofthe solder bonds 6, removing a manifold layer 41 with a grid 21 andattaching another manifold layer 41 with a grid 21 to the backside of asemiconductor chip 2.

In an alternative embodiment, the fabrication process described withreference to FIGS. 8A to 8H can also be carried out with a biggermanifold layer, which extends over both of the semiconductor chips 2when attached to the same (not shown).

In contrast to the multi-chip module 20 of FIG. 4, the multi-chip module40 depicted in FIG. 8H comprises two separate manifold layers 31. Afurther difference is that the manifold layers 41 are connected to thegrids 21 without a sealing.

FIG. 9 shows a plan view of the manifold layers 41 of the multi-chipmodule 40. Each of the manifold layers 41 comprises an inlet channel 44,an outlet channel 45 and channels 46 extending towards respectiverecesses 23 of the grids 21. The inlet and outlet channels 44, 45 areconnected to respective holes 43 of the top sealings 42 of the manifoldlayers 41 depicted in FIG. 8H, which serve as inlets and outlets. Thus,a flow of cooling liquid or water towards and away from the recesses 23of the grids 21 of connected fins can be established.

The manifold layers 41 can again comprise a rigid or flexible materialhaving a relatively low Young's modulus. In the case of a rigidmaterial, it is preferred that the manifold layers 41 comprises the samematerial as the semiconductor chips 2 in order to reduce differentthermal expansions of the manifold layers 41 and the semiconductor chips2.

In order to connect the separate manifold layers 41 and their inlet andoutlet channels 44, 45 with each other, the multi-chip module 40 canadditionally be provided with a connection layer 50 attached to themanifold layers 41, as illustrated in FIG. 10. The connection layer 50comprises an inlet 51 and an outlet 53. The inlet 51 is connected to aninlet channel 52 and the outlet 53 is connected to an outlet channel 54.The inlet channel 52 and the outlet channel 54 can be seen from the planview of the connection layer 50 depicted in FIG. 11. The inlet andoutlet channels 44, 45 of the manifold layers 41, as depicted in FIG. 9,are thereby connected to the inlet channel 52 and the outlet channel 54of the connection layer 50 via respective ones of the holes 43.

Concerning the cooling process, a cooling liquid 24 such as water flowstowards the recesses 23 of the grids 21 via the inlet 51 and the inletchannel 52 of the connection layer 50, the inlet channels 44 andrespective ones of the channels 46 of the manifold layers 41, and warmedcooling liquid 24 a flows away from the recesses 23 of the grids 21towards the outlet 53 of the connection layer 50 via respective ones ofthe channels 46, the outlet channels 45 and the outlet channel 54, whichcan be seen from FIGS. 9, 10 and 11.

The connection layer 50 can also comprise either a rigid or a flexiblematerial having a relatively low Young's modulus. In the case of a rigidmaterial, the connection layer 50 preferably comprises the same materialas the manifold layers 41 in order to reduce different thermalexpansions. As illustrated in FIG. 10, the connection layer 50 canadditionally be provided with a thinning 55 between the manifold layers41 in order to compensate geometrical imperfections.

FIGS. 12A to 12G illustrate another fabrication process to provide themulti-chip module 40 as an alternative to the fabrication processdescribed with reference to FIGS. 8A to 8H. This alternative fabricationprocess comprises the same or similar process steps in order to providea manifold layer 41 with a thermally conducting structure attached tothe same, e.g. a grid 21 of connected fins, which is depicted in FIG.12F.

In contrast to the previously described fabrication process withreference to FIGS. 8A to 8H, semiconductor chips 2 are mounted onto theupper surface of a common substrate 3 separately from the manifoldlayers 41, which can be seen from FIG. 12G. The semiconductor chips 2are again attached to the substrate 3 in a face-down state via solderballs 4 by utilizing e.g. a standard C4 technology process and compriseadhesion layers 5 coating the backsides.

Only subsequent to this process step, manifold layers 41 with grids 21are attached to the backsides of the semiconductor chips 2 via solderingin order to obtain the multi-chip module 40 depicted in FIG. 8H. Thus, amelting of the solder bonds 6 joined with an adhesion layer 5 due tomounting of a chip 2 onto the substrate 3 via soldering and a completeor partial dissolving of a grid 21 from the backside of thesemiconductor chip 2 as a consequence thereof, which might occur duringthe previously described fabrication process illustrated in FIGS. 8A to8H, is reduced.

Next, another aspect of the present invention will be described withreference to FIGS. 13 and 14.

FIG. 13 depicts a schematic cross-sectional view of a part of a manifoldlayer 41 attached to a structural element of a thermally conductingstructure, which is a stud 7, for instance, via a spring element 47 andFIG. 14 depicts a plan view of the same. By using spring elements 47connecting the manifold layer 41 to the structural elements of athermally conducting structure, the manifold layer 41 is mechanicallydecoupled from the thermally conducting structure.

The spring elements 47 can also be introduced between the manifoldlayers 41 and the grids 21 of the multi-chip module 40 of FIG. 8H andbetween the manifold layer 30 and the grids 21 of the multi-chip module20 depicted in FIG. 4, respectively. As a consequence, geometricalimperfections and different thermal expansions of e.g. a manifold layerand a thermally conducting structure can be at least partiallycompensated so that mechanical stress induction to semiconductor chipsand multi-chip modules, respectively, is further reduced. This allowsthe use of hybrid material systems with improved thermally capabilitiessuch as solders for enhanced thermally transfer and reliability.

In order to fabricate a semiconductor chip or a multi-chip module withsuch spring elements connecting a manifold layer to a thermallyconducting structure, a carrier comprising a manifold layer having astructured seed layer is provided in a first step. The structured seedlayer provides spring elements formed out of partitions of the seedlayer. As an example, the seed layer 11 of the manifold layer 41depicted in FIGS. 8A and 12A can be partially formed in such a way as toprovide spring elements. The subsequent processing steps are carried outsimilar to the fabrication processes described above with reference toFIGS. 8A to 8H and FIGS. 12A to 12G, respectively, and in such a waythat the structural elements of the thermally conducting structure, e.g.the fins of a grid 21, are attached to these spring elements.

The thermally conducting structures depicted in the preceding figuresall have structural elements with essentially vertical side walls.Furthermore semiconductor devices comprising thermally conductingstructures with different geometrical shapes are imaginable. Suchthermally conducting structures can also be formed and attached to asemiconductor device by utilizing one of the fabrication methodsdescribed above.

As an example, FIG. 15 depicts, in a cross-sectional view, asemiconductor chip 2 with a thermally conducting structure comprisingT-shaped fins 60 attached to the backside of the semiconductor chip 2via solder bonds 6. The T-shaped fins 60 are essentially formed of athermally conducting material like e.g. copper. The backside of thesemiconductor chip 2 is again coated with an adhesion layer 5.

The T-shaped fins 60 build reentrant cavities acting as vapor traps,which are known to reduce the superheat by a factor of ten for two phaseheat transfer systems. As a consequence, the semiconductor chip 2 shownin FIG. 15 or a multi-chip module comprising such semiconductor chips 2can be integrated into a device for two phase heat transfer, thusimproving the thermally performance and the critical heat flux.

While the present invention has been described in terms of specificembodiments, it is evident in view of the foregoing description thatvarious variations and modifications may be carried out withoutdeparting from the scope of the invention. As an example, theillustrated and described multi-chip modules and the respectivefabrication processes are not limited to multi-chip modules having onlytwo semiconductor chips.

Additionally, the fabrication methods described above are not limited toonly semiconductor chips and multi-chip modules, respectively. Thefabrication processes can also be carried out to transfer and attachthermally conducting structures formed on respective carriers to thebackside surface of a semiconductor wafer, which is subsequently dicedinto semiconductor chips.

Moreover, the fabrication methods described above may also be used tomanufacture other functional components on a carrier and to transfer andattach these components to the backside of a semiconductor device. Suchfunctional components can e.g. comprise micro-electromechanical (MEMS)devices such as valves or pumps. Furthermore, the fabrication methodscan be utilized to stack layers on the surface of a semiconductordevice.

1. A semiconductor device comprising: a thermally conducting structureformed on a carrier and attached to a surface of the semiconductordevice via soldering, said thermally conducting structure beingessentially formed of a thermally conducting material and comprising: aplurality of structural cooling elements of a high aspect ratio selectedfrom a group consisting of: an array of freestanding fins, studs, andframes and a grid of connected fins; a very thin layer of solderingapplied between the thermally conducting structure and the surface ofthe semiconductor device; wherein the very thin layer is less than 1 μm;a cooling liquid; a manifold layer for supplying the cooling liquid,said manifold layer comprising fluid inlets and outlets; an adhesionlayer coating the surface of the semiconductor device for attaching thethermally conducting structure;
 2. The semiconductor device of claim 1wherein said manifold layer is attached to the thermally conductingstructure via spring elements.
 3. The semiconductor device of claim 1comprising at least two semiconductor chips attached to a commonsubstrate, said thermally conducting structure being attached to thesurfaces of the semiconductor chips.
 4. The semiconductor device ofclaim 3, wherein the manifold layer is attached to the thermallyconducting structure after removing the carrier, said manifold layercomprising a thinning between the semiconductor chips.
 5. Thesemiconductor device of claim 1 further comprising a device for jetimpingement cooling integrated into said semiconductor device.
 6. Thesemiconductor device of claim 5 wherein the device for jet impingementcooling comprises nozzles for dispensing a cooling liquid through theinlets to the manifold layer.
 6. The semiconductor device of claim 1wherein the plurality of structural cooling elements are formed ofcopper.
 7. The semiconductor device of claim 3 wherein the manifoldlayer comprises an electro-conductive seed layer.
 8. The semiconductordevice of claim 7 wherein the carrier is in a form of a transparentsubstrate coated with a polyimide layer, the seed layer being formed ontop of the polyimide layer.